The invention relates to a field-device coupling unit for providing a supply voltage for a field device and for communication with a superordinate control unit, the field-device coupling unit comprising a current interface which is configured for communication with the control unit and has a first controllable input load which is configured to provide, from an interface current of the current interface, a load voltage on which the supply voltage is based, the field-device coupling unit further comprising a circuit arrangement for providing a communication signal to be transmitted to the control device.
A field-device coupling unit of the type described above is known from the prior art and is used to connect a field device to a superordinate control unit via a current interface.
The field-device coupling unit fulfils primarily two functions. Firstly, the field-device coupling unit is used to provide a supply voltage for the field device based on the interface current of the current interface. Secondly, the field-device coupling unit makes it possible to communicate with the control unit by means of the current interface.
In order to fulfil the first function—i.e. providing a supply voltage—the known field-device coupling unit comprises a controllable input load which is preferably connected in series in the current interface, and the load voltage of which—i.e. the voltage which drops over the input load—is the basis of the supply voltage; that is, said load voltage is used as a supply voltage or as a basis for the supply voltage. The controllable input load is intended to ensure in particular that the load voltage remains within a specific voltage range so that said voltage can be used as a supply voltage or as a basis for the supply voltage. Depending on the configuration of the controllable input load, said input load can also meet specific requirements for explosion protection, for example in that currents and/or voltages associated with the input load are limited to such an extent that a minimum ignition energy of an explosive mixture is not reached.
As mentioned above, the second function of the field-device coupling unit consists in allowing communication between the field device and the superordinate control unit by means of the current interface. In this context, the basic principle of the communication shall firstly be explained. Conventionally, for communication by means of a current interface, the communication signal to be transmitted has been mapped onto the current level of the interface current. However, since, in this way, only very low data-transmission rates can be achieved, a transmission process has been developed in which the interface current is superimposed with a frequency-modulated current signal which contains the information to be transmitted. For example, the HART (highway addressable remote transducer) protocol is based on a transmission process of this type. Here, according to the principle of frequency shift keying, various transmission symbols are mapped onto various frequencies which the current signal assumes sequentially over time. In this case, the frequencies are outside the normal spectrum of the interface current, and therefore the current signal is not disrupted by changes in the interface current.
In order to now superimpose said current signal on the interface current, in conventional field-device coupling units, specific superposition circuits are provided. Said superposition circuits increase the circuit complexity of the field-device coupling unit, as a result of which said unit is more difficult to produce, more complex and more prone to errors.